Integrated circuit chips are typically formed on semiconductor wafers. Various layers of different materials are built on the wafers in various different types of chambers. Such chambers can include rapid thermal processing chambers and chemical vapor deposition chambers. In a chemical vapor deposition chamber, a gas or vapor is fed into the chamber which reacts with the surface of the wafer.
A rapid thermal processing chamber, which can be used as a chemical vapor deposition chamber, refers to a device that rapidly heats objects, such as semiconductor wafers. Such devices typically include a substrate holder for holding a semiconductor wafer and an energy source for heating the wafer. During heat treatment, the semiconductor wafers are heated under controlled conditions according to a pre-set temperature regime. For monitoring the temperature of the semiconductor wafer during heat treatment, thermal processing chambers also typically include radiation sensing devices, such as pyrometers, that sense the radiation being emitted by the semiconductor wafer at a selected wavelength. By sensing the thermal radiation being emitted by the wafer, the temperature of the wafer can be calculated with reasonable accuracy.
Many semiconductor heating processes require a wafer to be heated to high temperatures so that various chemical and physical transformations can take place as devices are fabricated on the wafer. During rapid thermal processing, which is one type of processing, semiconductor wafers are typically heated by an array of lights to temperatures, for instance, from about 400xc2x0 C. to about 1,200xc2x0 C., for times which are typically less than a few minutes. During these processes, one main goal is to heat the wafers as uniformly as possible.
In order to heat wafers as uniformly as possible, the wafers are typically rotated within the thermal processing chamber. Rotating the wafer promotes greater temperature uniformity over the surface of the wafer and promotes enhanced contact between the wafer and any gases introduced into the chamber.
In the past, various mechanical systems have been used in order to rotate wafers in thermal processing chambers. Unfortunately, however, the mechanical systems have a tendency to generate small particles caused by the mechanical parts contacting each other. These particles can enter the chamber and contaminate the process being carried out. Even the slightest amount of contamination within the chamber can drastically affect the properties of the wafer or of layers being formed on the wafer.
As such, a need currently exists for an improved process and system for rotating wafers in thermal processing chambers, such as rapid thermal processing chambers and chemical vapor deposition chambers. In particular, a need exists for a system and process for rotating wafers in thermal processing chambers that efficiently rotate the wafers without the risk of contaminating the processing chamber.
The present invention is generally directed to a method and system for processing semiconductor wafers in thermal processing chambers. More particularly, the present invention is directed to magnetically levitating and magnetically rotating semiconductor wafers during processing.
For example, in one embodiment, the system of the present invention includes a thermal processing chamber adapted to contain semiconductor wafers. A heating device, such as a plurality of energy sources, are positioned outside the chamber for heating the semiconductor wafers contained within the chamber. A rotatable substrate holder is positioned within the thermal processing chamber and is configured to support a wafer being processed.
According to the present invention, the system further includes a rotor supporting the substrate holder. The rotor can have a circular shape and can be made partially or completely of a material capable of being influenced by a magnetic force. At least one suspension actuator is positioned outside of the chamber and above, below or at an angle to the rotor for levitating the rotor. In order to rotate the rotor, the system further includes a plurality such as at least three rotation actuators also positioned on top, on the side or below the rotor outside of the chamber. The suspension actuator and the rotation actuator each are capable of generating a magnetic field for levitating and rotating the rotor respectively.
In one embodiment, the rotation actuator includes a C-shaped magnetic element having a pair of opposing poles that define first and second rotation surfaces. The C-shaped magnetic element is placed in operative association with a magnetic coil that generates a magnetic field when an electric current is fed through the coil. The rotor can be positioned in between the first and second rotation surfaces.
The rotor can have a smooth surface or can include a plurality of spaced apart radial teeth. The teeth can be positioned in between the first and second rotation surfaces. In this embodiment, the rotation actuator can create a pulsing or variable magnetic field that acts upon the radial teeth to rotate the rotor.
Besides using a rotation actuator which produces a variable magnetic field through the use of a magnetic element and a magnetic coil, in an alternative embodiment of the present invention, the rotation actuator can be a rotating disk that contains a plurality of permanent magnets. The disk can be positioned so that the edge of the disk is located adjacent to the rotor as the disk is rotated. The disk can be rotated using, for instance, a motor. Permanent magnets can be installed on the periphery of the disk with the magnetic pole ends in the radial direction. In this manner, as the disk is rotated, the permanent magnets induce a field in the rotor. The induced field causes an attractive force to be set up between the rotor and each rotating permanent magnet. As the magnet turns and moves away from the induced field of the rotor, the attractive force results in a torque which causes the rotor to rotate. One or more of these disks containing the permanent magnets can be placed adjacent to the rotor as desired.
As mentioned above, the system of the present invention includes at least one rotation actuator. In one embodiment, the system can include a plurality of rotation actuators, such as from about three rotation actuators to about twelve or more rotation actuators. The rotation actuators can be positioned around the rotor at any desired location.
The suspension actuator used in the present invention, in one embodiment, can include a U-shaped magnetic element surrounded by a coil. Similar to the rotation actuator, the magnetic element can generate a magnetic field when an electric current is fed through the coil.
The U-shaped magnetic element of the suspension actuator can include a first suspension surface and a second suspension surface that face the rotor. The rotor, in turn, can include first and second annular raised portions positioned below the first and second suspension surfaces. When the suspension actuator creates a magnetic field, the rotor can be levitated through the attraction of the first and second annular raised portions with the first and second suspension surfaces. Through this configuration, not only is the rotor levitated, but is also maintained in radial alignment due to the presence of both of the annular raised portions and the reluctance centering relationship they form with the suspension actuator.
In one embodiment, the system can include position sensors located adjacent to the rotor. The position sensors can monitor the vertical position of the rotor in relation to a horizontal plane. In one embodiment, the system can include two vertical position sensors, one being the reference sensor. The position sensors can be placed in communication with a controller, such as a microprocessor. The controller can be configured to receive information from the position sensors regarding the position of the rotor and, based on the information received, to independently adjust each suspension actuator included in the system for levitating the rotor a determined distance and for maintaining the rotor parallel to the horizontal plane. Besides being controlled independently, the suspension actuators can also be controlled in coordination with each other. Coordinating control can be implemented in a multi-input-multi-output control scheme.
Besides vertical position sensors, the system can also include rotation sensors. For example, a first rotation sensor can be used to monitor the speed of the rotor, while another rotation sensor can be used for homing position. Examples of rotation sensors that may be used in the present invention include Hall Effect sensors or laser sensors.
Other features and aspects of the present invention are discussed in greater detail below.